The most popular flat panel display technology currently in use is based on liquid crystal devices, which are effectively light shutters used in combination with illumination sources. One of the particular advantages of liquid crystal displays is that the power required to switch the light shutters on and off is relatively low and if the light source used to provide illumination is an ambient one as used in reflective LCD technology, then overall power consumption for the display can be very low. Therefore the technology has great potential for low power portable applications e.g. personal organisers. There are two problems, however. One is the poor angle of view, though for portable applications this is not typically a severe constraint. More importantly, the ability to get full colour displays at reasonable contrast is very limited in reflective displays.
Organic electroluminescent devices are made from materials that emit light when a suitable voltage is applied across electrodes deposited on either side of the organic material. One class of such materials is semiconductive conjugated polymers which have been described in our earlier U.S. Pat. No. 5,247,190, the contents of which are herein incorporated by reference. The power requirements, however, of conjugated polymer EL devices in common with other emissive technologies are all relative high compared to reflective LCD.
The readability of a display depends on many factors but contrast and brightness are the most important. A display can be bright, but not easily readable because the contrast with respect to the non-display area is poor. This can certainly be the case in high ambient light conditions, where reflection of light from both display and non-display areas reduces the contrast of the display. This problem is most common in dynamic displays, i.e. in those displays where certain areas may be switched on or off depending on the nature of the information to be displayed. Conversely a display with high contrast in high ambient light conditions can be readable at a relatively low brightness.